1. Field of the Invention
The present invention relates to a gas sensor for measuring a particular gas component of exhaust gas from, for example, an internal combustion engine of an automobile.
2. Description of the Related Art
Conventionally, gas sensors have been utilized for detecting particular gas components, such as nitrogen oxides (NOx) and oxygen, and for measuring the concentrations of particular gas components.
Among these gas sensors, a gas sensor is known that uses an elongated-plate-like gas sensor element configured such that a plurality of ceramic layers (e.g., solid electrolyte layers and alumina substrates) are laminated.
A known technique that utilizes through-holes extending through laminated ceramic layers is used for electrically connecting internal conductors (e.g., a heat-generating resistor and electrodes) of the gas sensor element to corresponding electrode pads provided on the surface of the gas sensor element.
For example, Patent Document 1 mentioned below discloses a plate-like gas sensor element in which a plurality of solid electrolyte substrates, etc., are laminated together.
Specifically, a gas sensor having the above-mentioned gas sensor element is mounted to, for example, an exhaust pipe of an internal combustion engine of an automobile. As shown in FIG. 9, a gas sensor element P1 is a plate-like laminate of a first pump cell P5 in which a pair of first electrodes P3 and P4 is provided on a first solid electrolyte body P2, a second pump cell P9 in which a pair of second electrodes P7 and P8 is provided on a second solid electrolyte body P6, an oxygen concentration detection cell P13 in which a pair of third electrodes P11 and P12 is provided on a third solid electrolyte body P10, etc.
Furthermore, the gas sensor element P1 has a reference oxygen chamber P14 between the second solid electrolyte body P6 and the third solid electrolyte body P10. In the reference oxygen chamber P14, one electrode (second outer electrode) P8 of the two second electrodes and one electrode (reference electrode) P12 of the two third electrodes are disposed opposite each other. Furthermore, in order to prevent contact between the two electrodes P8 and P12, the reference oxygen chamber P14 has an insulative protection layer P15 formed from an electrically insulative porous material between the two electrodes P8 and P12.
Furthermore, the electrodes P3, P4, P7, P8, P11, P12 are electrically connected to corresponding electrode pads P22 (see FIG. 10) provided on the outer surface of a rear end portion of the gas sensor element P1, via leads and through-holes.
As shown in FIG. 10, in a gas sensor P16, the gas sensor element P1 is disposed so as to extend through a ceramic holder P19, talc P20, and a ceramic sleeve P21, which are disposed within a through-hole P18 of a metallic shell P17, and is fixedly united with these members by means of a rear end portion of the metallic shell P17 being crimped.
Furthermore, the electrode pads P22 provided on the outer surface of a rear end portion of the gas sensor element P1 are in contact with corresponding metal terminals P24 which are connected to respective lead wires P23 extending from an external circuit (not shown), whereby signals of the gas sensor element P1 are output to the external circuit.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2010-266429
3. Problems to be Solved by the Invention
Meanwhile, in operation of an internal combustion engine of an automobile or the like, exhaust gas from the engine may cause water, such as water vapor contained therein and condensed water adhering to the inner surface of an exhaust pipe, to adhere to the gas sensor P16. The adhering water may cause a problem in the gas sensor P16 as discussed below.
In the gas sensor P16, the above-mentioned members such as talc P20 isolate a forward end portion of the gas sensor element P1 which is exposed to exhaust gas, from a rear end portion of the gas sensor element P1 where the electrode pads P22, etc., are exposed. However, water may creep along the surface of the gas sensor element P1 (in the direction of the arrow of FIG. 10) from the forward end portion to the rear end portion.
Furthermore, water which has reached the rear end portion of the gas sensor element P1 may permeate along through-holes connected to the corresponding electrode pads P22 exposed at the rear end portion and then along leads disposed between ceramic layers, and may finally reach the electrodes disposed within the forward end portion of the gas sensor element P1.
At this time, if water reaches the reference oxygen chamber P14 via the second outer electrode P8 and the reference electrode P12, water may permeate through the porous insulative protection layer P15. The permeating water may impair insulation between the opposed electrodes P8 and P12 so as to short-circuit the same, and potentially result in hindering gas measurement by the gas sensor P16.